Every light source has four basic features that are relevant for the purposes of the project. Two of these influence the aesthetic characteristic of the system, while two are of an economic character. Both groups include, respectively, colour temperature, colour rendering index, luminous efficiency, and durability (or average life). While the former produce a more or less "warm" light and have a varying impact on colour perception, the latter influence energy consumption and running costs (including the cost for source replacement). These four features differ in each lamp; likewise incandescent lamps (by now obsolete), incandescent halogen lamps, fluorescent and metallic iodide lamps, and LEDs show major differences, also in their physical appearance. Practically speaking, small-sized sources (e.g. halogen lamps) are best suited for projectors and whenever light needs to cover a long distance (4-5 metres or more from the source) and to clearly outline the illuminated area, creating light and shade, or to achieve the warm, brilliant atmosphere, typical of these lamps. Fluorescent lamps help achieve different hues of light, from warm pink to cold blue, and are suited for general illumination, for soft diffused light, or to minimize consumption. Metal iodide lamps are comparable to fluorescent lamps in terms of efficiency and, like these, allow to choose among three different colour temperatures and, therefore, to obtain warmer or colder light hues; they also provide some of the characteristics of halogen lamps, i.e., they are suited for projectors to obtain narrow and sharply defined light beams.
LED appliances – in constant evolution – provide benefits in terms of limited size and opportunity to change both the light intensity and the colour, ensure high efficiency, and have an extremely long life, unlike traditional ones.
Incandescent lamps are traditional lamps with Edison E27/E14 screw lamp holders. More than a century after their invention, they are now being dismissed due to their high inefficiency, close to 10 lumens/Watt. Their nominal life is about 1000 hours, and they are sensitive to impacts and vibrations. When lit they quickly reach a temperature of up to 600° C and should not be touched with fingers or easily inflammable objects. The colour of their light is a 2700 K warm white; they enhance red hues, but fade blue nuances. They are simply powered by connecting them to the power mains.
Incandescent lamps with halogens
Halogen lamps are made to withstand higher temperatures than normal incandescent lamps; moreover the bulb contains an element (halogen) that prevents blackening of the same. Such halogen lamps are smaller than common incandescent lamps with the same power, but ensure more efficiency and a longer average life. Some of the most popular types have a built-in reflector, which allows to direct the light flow. The appropriate light cone width and power can be chosen from a catalogue. Linear and simple bulb types of lamps can be connected to the power system, while the more common lamps with projector require a 12V energy supply and consequently a 240V/12V transformer.
Compact fluorescent lamps, also known as “energy saving lamps” are part of the family of fluorescent lamps, and were recently improved in terms of light hue and colour rendering. The use of fluorescent lamps for home illumination is therefore recommended not just because of their luminous efficiency and long life, but also for their versatility. Fluorescent lamps require a special electronic energy supply circuit that removes the stroboscopic effect, increases the lamp’s efficiency and average life, and allows prompt switching, crucial in a house.
Metal halide lamps
Metal halide lamps are almost as cheap to use and as durable as fluorescent lamps, but are also much more compact and have almost the same size as halogen lamps. They are suited to produce direct and indirect illumination, with close flow concentration achieved without consuming undue amounts of energy. They require a complex power supply system. Their main drawback is that they switch on gradually (reaching maximum performance in about 5 minutes) and above all, they are slow to restart. If accidentally switched off, peak performance is only reached about 10 minutes after restarting. They are generally not too popular for home applications due to their long start/restart time and to the heat they produce.
A Light Emitting Diode (LED) is an electronic unit that generates a particular colour of light when energy runs across it. White light is obtained using a technology similar to fluorescent tubes. Phosphor is added to a blue LED: the light of phosphor and the light of the blue LED are blended together to generate white light with different colour temperatures.
The efficiency of LEDs is by now comparable to discharge lamps. They ensure high colour rendering (technology is constantly improving in this respect) and a broad range of colours and colour temperatures. Their average life is much longer compared to all traditional lamps, they have a very limited size, but require heatsinks and power and control systems that can also be operated remotely.
The main drawback of LEDs is that they are still quite expensive.